How to Buy a Telescope : Telescope Buying Guide: Composite Telescopes

Another type of reflecting telescope is a
compound telescope, most commonly referred to as an SET or a Schmidt-Cassegrain. Now
the Schmidt-Cassegrain uses a lens as well as a mirror to gather light, so as you can
see, as we’re gazing skyward, the light is coming in through the lens here and it’s bouncing
off a mirror at the back of the rear cell, up to another primary mirror here, down to
the eyepiece in the back of the telescope. One of the advantages of that design is it
gives us a longer focal length and a more compact design. I’ll show you right over here,
we’ve got an eight inch Newtonian with a thousand millimeter focal length and here is a eight
inch Schmidt-Cassegrain with a two thousand millimeter focal length. The light is traveling
further distance in this telescope than it is in that one right there. So again, that’s
important to know when I’m choosing my eyepiece. My 22 millimeter eyepiece in this telescope
will give me 96 magnification, in that telescope it gives me 45 magnification. Both good powers
for astronomy, but both two things you need to know about the focal length and choosing
which telescope you’re going to choose. Another advantage obviously to the eight inch is it’s
more portable. A telescope like this–the Schmidt-Cassegrain–packs right up. I lock
down the clutch there I can unscrew it from the tripod, pick it right up, and go out the
door. If I’m just in a garage, I’ve got a clear space, I can pick it up–tripod and
all–and take it outside. So up to an eight inch size they’re fairly portable. Ten is
probably about the biggest I’d want to go buy myself for hauling it around. You don’t
want to get too much telescope. Again, I keep saying aperture’s the most important, but
if your sky suddenly clears up at eight, nine o’clock at night, and you want to go out stargazing
but your scope is too much to haul out, you got the wrong telescope. So, keep that in
mind when you’re purchasing a telescope and consider a Schmidt-Cassegrain as well. Again,
not ideal for daytime use, but the smaller sizes–four, five, six inch–you can get usable
power for daytime use. Not typically what I would sell you for a daytime telescope,
but it’ll double fairly well there as well. Again, the advantage here mostly is portability,
good contrast–again, that’s important to make those dim fuzzies pop out–and also ideal
for astral photography as well.


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